The present invention is a method for controlling the direct process product distribution to vary the concentrations of trialkylhalosilanes and alkyltrihalosilanes in the product. The method comprises contacting an alkyl chloride with silicon metalloid in the presence of a catalytic amount of a catalyst comprising 0.5 to 10 weight percent copper, 5 to 200 ppm tin, 25 to 2,500 ppm phosphorous, and greater than zero to less than 50 ppm zinc, where the concentration of each of the catalyst components is based on the weight of the silicon, at a temperature within a range of about 250.degree. C. to 400.degree. C. The present inventors have found that by controlling the zinc concentration the concentrations of triatkylhalosilanes and alkyltrihalosilanes in the product can be varied. The present method is especially useful for increasing the concentration of trialkylhalosilane in the product.
The present invention relates to a modification of what is commonly referred to as the direct process for producing alkylhalosilanes, where the process comprises contacting an alkyl halide with particulate silicon in the presence of a copper catalyst. The process was first described by Rochow in U.S. Pat. No. 2,380,996 and U.S. Pat. No. 2,380,995.
Since the original description of the direct process by Rochow, the process has been refined and modified in numerous ways and is used for producing virtually all commercial alkylhalosilanes in the world today. When one considers that several million pounds of silanes are produced annually and consumed by the silicones commercial efforts, it is obvious that even small changes in the product distribution from the direct process can be important.
Commercially, the largest volume alkylhalosilane manufactured is dimethyldichlorosilane, as this alkylhalosilane constitutes the backbone of most high volume commercial silicone products after it has been hydrolyzed and condensed to form silicone polymers. Therefore, most efforts in optimizing the direct process have been directed toward obtaining the highest yield of dialkyldihalosilane. At times commercial demand for other alkylhalosilanes, particularly trialkylhalosilane, can exceed what is being produced in the direct process as optimized for production of dialkyldihalosilane. Current processes such as redistribution of dialkyldihalosilane to form trialkylhalosilane and alkyltrihalosilane are expensive to perform making then commercially unattractive. The present process provides a method for controlling the direct process to vary the concentration of trialkylhalosilane and alkyltrihalosilane in the product, thereby making the amounts of trialkylhalosilane and alkyltrihalosilane in the product more closely match commercial demand.
Ward et al., U.S. Pat. No. Re. 33,452, describe a process for making alkylhalosilanes by effecting reaction between an alkyl halide and powdered silicon in the presence of a copper-zinc-tin catalyst. Ward et al. suggest that improvements in reaction rate and product selectivity are achieved when copper is employed at a critical weight percent relative to silicon and critical weight ratios of tin and zinc are employed relative to copper. Ward et al. do not teach the presence of phosphorous or phosphorous compounds as part of the catalyst mixture.
Halm et al., U.S. Pat. No. 4,602,101, teach a method of controlling a process for the preparation of alkylhalosilanes from silicon and alkylhalides where phosphorous or phosphorous compounds are used as promoters to enhance selectivity of the process for dialkyldihalosilanes. Halm et al. teach that the catalyst can comprise 0.2 to 10 weight percent copper based on silicon present, 5 to 200 parts per million (ppm) tin, 25 to 2500 ppm phosphorous and optionally 100 to 10,000 ppm zinc.
Freeburne et al., U.S. Pat. No. 5,312,948, teach that a preferred catalyst for the reaction of silicon metalloid with an alkyl halide to form alkylhalosilanes comprises on an elemental basis by weight: 0.1 to 10 weight percent copper based on silicon present in the process, 50 to 10,000 ppm zinc, 5 to 200 ppm tin, and 25 to 2,500 ppm phosphorous.
The above described art does not teach conduct of the direct process in the presence of a catalyst mixture providing on an elemental basis by weight 0.1 to 10 weight percent copper, 5 to 200 ppm tin, 25 to 2,500 ppm phosphorous, and greater than zero to less than 50 ppm zinc, where the concentration of each of the catalyst components is based upon the weight of silicon in the process. Furthermore, the described art does not recognize that the concentration of zinc in the direct process can be controlled as a method of varying the concentration of trialkylhalosilane and alkyltrihalosilane in the product.